THE REACTION OF NORMAL AND ABNORMAL NERVE 149 



Clearly, this law is applicable only to excised nerve and muscle 

 when tested under experimental conditions, but its practical value 

 will become apparent later on in connection with the stimulation of 

 normal and degenerating human muscle and nerve. Its explanation 

 will present no difficulties if the following three fundamental data are 

 borne in mind, namely: 



(a) When a nerve is stimulated with a galvanic current, an excitatory process is 

 set up at the cathode on the making and at the anode on the breaking of the 

 current. 



(6) The excitatory condition developed at the cathode on the making, is 

 stronger than the one generated at the anode on the breaking of the current. 



(c) The passage of a galvanic current through a nerve entails a decrease in its 

 power of conduction which, although discernible at both poles, is most strongly 



t 1 



i t 



i T 



FIG. 88. DIAGRAM ILLUSTRATING PFLUGER'S LAW. 



Asc, ascending current; Desc, descending current; W, M, <S, weak, medium and 

 strong current. The effective stimulus is indicated in each case by cross marks. 



marked in the region of the anode. Immediately upon the breaking of the current, 

 the anodic conductivity returns to near normal, while the cathodic conductivity is 

 diminished. With strong currents this anodic depression on the making becomes 

 so powerful that it actually blocks the nerve impulse and thus prevents it from 

 reaching its destination. The same holds true of the cathodic depression resulting 

 after the breaking of the strong constant current. 



With a weak ascending or descending current, only the two making 

 stimuli are effective, because in this case the excitation which gives 

 rise to the nerve impulse is developed at the cathode (Fig. 88). The 

 nerve impulse resulting therefrom, reaches the muscle, because the 

 depression at the anode on the making of the ascending current is not 

 sufficiently intense to block it. The same holds true of the making 

 of the descending current, and besides, the stimulating cathode now 

 lies next to the muscle, so that nothing can prevent the passage of 



